1. Field of the Invention
The present invention relates to an optical transmission system which transmits signal light in which a plurality of signal channels with optical frequencies different from each other are multiplexed.
2. Related Background Art
A wavelength division multiplexing (WDM) optical transmission system is an optical system which transmits, through an optical fiber transmission line, signal light (WDM signal light) in which a plurality of signal channels with optical frequencies different from each other are multiplexed. The WDM optical transmission system can transmit/receive a large quantity of information at a high speed. For the optical transmission system as a backbone network with a great demand for communication, a further increase in capacity has been examined by reducing the optical frequency spacing of a plurality of signal channels to increase the degree of multiplexing. WDM with a higher degree of multiplexing is called a DWDM (Dense WDM).
On the other hand, in an optical transmission system with not so large demand for communication, the degree of multiplexing is decreased by increasing the optical frequency spacing of a plurality of signal channels, thereby reducing the system cost. WDM with a lower degree of multiplexing is called a CWDM (Coarse WDM). In a CWDM optical transmission system, as the number of signal channels in signal light decreases, the number of optical components (e.g., signal light sources, light-receiving elements, and the like) is reduced. In addition, inexpensive optical components (e.g., optical multiplexers, optical demultiplexers, and the like) whose wavelength accuracy requirement values are small because of the large optical frequency spacing are used. Accordingly, the system cost can be reduced.